in this situation, you should not tell the prime tenant or the landlord, but hire a lawyer to sue in supreme court to have the prime tenant declared to be an illusory tenant. you will be able to collect any rent overcharge from the prime tenant and have the court order the landlord to accept you as the tenant of the apartment and the landlord will be ordered to offer you a lease in your name. so, you have found yourself in a fortoniate situation, which you can take advantage of, if you paly it right. tipping off the prime tenant or the landlord of your awareness of the illusory tenancy situation -- before you sue the prime tenant -- will backfire on you!! good luck...need a good tenant lawyer,,,,hire me